This invention relates to filtration and dewatering of sludge and other slurry. Particularly it relates to an apparatus employing continuous filter-dewatering-expression under vacuum-compression.
Sludge may be defined as a semiliquid deposit or sediment having a total solids concentration of at least 2500 ppm.
Sludge handling and disposal is the most troublesome aspect of wastewater treatment, and it is often the most costly. The problem is increased with efficient wastewater treatment plants being built and operated today producing higher fractions of the waste impurities in the form of sludges than formerly were possible. Also, in advanced wastewater treatment, the use of chemicals produces great quantities of sludge, further complicating the sludge handling and disposal problem.
One step in sludge handling and disposal involves filtration or dewatering, to reduce the sludge moisture content, generally to a non-fluid form, to a degree which allows ultimate disposal by incineration, landfilling, or other methods. Heretofore, sludge filter-dewatering has been attempted principally by open-air bed drying, vacuum filtration, centrifugation, and mechanical separation. A brief comment on each of these techniques may be helpful to an understanding of the purpose and achievements of the present invention.
Open-air drying beds have become less and less attractive, due to reduced land availability and the lessening of public tolerance for techniques open to the atomsphere with the possibility of air pollution.
Vacuum filtration separates the insoluble solids from the liquid by using differential pressures created by a vacuum pump, to force the liquid through a porous medium carried on a rotating drum, on which the solids remain to form a cake. The filter medium, which requires replacement periodically, requires a substantial flow of washwater to prevent plugging or blinding of the open area. The vacuum filter involves large equipment, many moving parts, high operating horsepower, and is expensive to purchase, operate and to maintain.
Centrifuges require high speeds of revolution, typically ranging from approximately 2000 rpm to 4500 rpm. The insoluble solids of the feed slurry or sludge centrifugally settle against the bowl wall, and the liquid flows toward and out a central discharge. The solids are typically pushed out through discharge ports and collected from the bowl. The centrifuge cannot tolerate grit without excessive wear and requires large operating horsepower. They are expensive to purchase, to operate, and to maintain.
Mechanical separation has typically been sought by plate and frame filters, typically comprising an alternating series of empty rings that serve to contain the final filter cake. The plates are covered on both sides with cloth and are provided with drainage channels for removal of the filtrate. The frames are filled with a batch of slurry under pressure, and the filtrate escapes through the cloth and through the plate channels, while the frames are left filled with filter cake. At the end of each cycle, the press is opened and the cake is removed for further treatment or disposal. This is an intermittent or batch type of operation and therefore is associated with the high labor costs resulting from manual operation, so that the plate and frame filters are expensive to operate and very expensive also in first or capital costs.
A typical dewatering screw press of the prior art comprised a rotating screw fitting closely inside a horizontal slotted or perforated curb. The curb and screw were typically tapered toward the discharge end, in order to increase the pressure on the material being dewatered. The discharge end was partially closed by an adjustable cone to vary the discharge opening and thus to vary the pressure on the material within the dewatering press. Rotation of the screw conveyed the material toward the discharge end, and as the pressure increased, the liquid associated with the material was forced out through the slots or perforations in the curb. However, it has been found that the dewatering press of the prior art will not perform satisfactorily on sewage sludges, due to the plugging and blinding of the slots or perforations of the filter-dewatering medium or curb, by the small and stringy solids contained in sewage sludge. The prior art did not provide an effective method or apparatus for cleaning the slots or perforations without interrupting continuous dewatering. Also, the prior art of the dewatering press did not provide for a reduction of the initial or design open area or the perforations or slots in the curb without a complete change of the curb itself.
My earlier U.S. Pat. No. 3,695,173, issued on Oct. 3, 1972, provides an improved apparatus for filtration-dewatering of sludge continuously. It has a permanent filter-dewatering medium comprising a series of hoops or rings separated and closely spaced from each other and held together as a single unit by a frame, and apparatus for continuously cleaning the filter-dewatering medium while filtering or dewatering. To prevent plugging or blinding and interruption of continuous filtration or dewatering, cutter or slot cleaning blades are secured to the outer edge of the screw conveyor, projecting out radially and spaced to extend into the slots or spaces between the closely spaced filter rings or hoops. The fine solids which have entered and become wedged in the spaces between the filter rings tending to plug or blind the open area of the filter-dewatering medium and the small stringy portions of large solids which have entered the spaces between the filter rings, but cannot pass through and out due to the large portions of the solids being retained on the surface of the filter-dewatering medium, are dislodged and pass through and out with the filtrate with the cutting or cleaning action of the cutter or slot-cleaning blades, when the rotating helical blade of the screw conveyor along with the conveyed sludge or dewatered solids, cuts and scrapes away the solids retained on the filter-dewatering medium.
U.S. Pat. No. 3,695,173 also provides apparatus to decrease the initial or design open area of the filter-dewatering medium to prevent undue bleeding of the solids into the filtrate by the utilization of slot-reducing or space-bridging members or material which bridge or partially close or effectively reduce the open area or spaces between the closely spaced filter rings of the filter-dewatering medium when the sludge nature and consistency warrants.
An important object of this invention is to provide a greatly improved apparatus for filter-dewatering-expression of sewage sludge or other slurries. One aspect is directed to improved apparatus for preventing plugging and blinding of the filter-dewatering medium and interruption of the continuous filtration, dewatering, and expression.
A characteristic of the prior art dewatering screw presses was to employ an adjustable choke or cone at the discharge end of the press to vary the discharge opening and control the back pressure within the pressing chamber. It has been found, however, that for dewatering sewage sludge, the adjustable choke or cone at the discharge end of the dewatering press, by itself, will not provide for sufficient variation in back pressure within the press. U.S. Pat. No. 3,518,936 issued to Bredeson provides for adjustably controlling the backup pressure within the press at one or more points intermediate the inlet and discharge ends of the press with annular choke members. However, it has been found that while the intermediate choke does increase pressure upon the material at the point of application of the choke, after the material has been conveyed past the choke, the pressure will be reduced. Due to the large variation in the nature and consistency possible with sewage sludge, this cyclic increase and decrease in pressure upon the sludge has been ineffective in adjusting the back pressure within the press. The prior art does not provide for varying the inside configuration of the dewatering press to a great extent, decreasing the dewatering and compression volume within the press, nor changing the dewatering and compression rate within different portions of the press.
The present invention has the further object, therefore, of providing an apparatus for increasing the diameter of the combination sludge compression and dewatered solids discharge screw conveyor shaft, varying the inside configuration of the dewatering press, decreasing the compression volume within the press, changing the compression and dewatering rate within different portions of the dewatering press, and at the same time, decreasing the dewatered solids outlet, thereby varying the quantity and dryness of the dewatered solids discharged from the press.
A further object of this invention is to provide for regulating the quantity and dryness of the dewatered solids discharged from the press.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.